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BOSTON—Joseph Kennedy III said Wednesday he's formally jumping into the race for the congressional seat now held by retiring U.S. Rep. Barney Frank.

"I believe this country was founded on a simple idea: that every person deserves to be treated fairly, by each other and by their government, but that's not happening in America anymore," Kennedy, a Democrat, said ahead of an announcement scheduled for Thursday.

He said he would work hard to earn every vote and if elected would "bring that fight for fairness to the U.S. Congress."

Kennedy, the son of former U.S. Rep. Joseph Kennedy II and a grandson of the late Robert F. Kennedy, recently moved from Cambridge to Brookline, part of the state's newly redrawn 4th Congressional District.

The family has deep ties to the Boston suburb. Brookline is the site of the birthplace of Kennedy's great-uncle, the late President John F. Kennedy.

Kennedy announced last month he was forming an exploratory committee to look at a possible run for the seat. He isn't the only candidate in the race, though.

Republican Sean Bielat, who lost to Frank, a Democrat, in 2010, also has jumped into the contest. He made his announcement official in January.

"I think we started something last time," Bielat, a former U.S. Marine who lives in Norfolk, said at the time. "I think we made some big gains, but we didn't get the job done."

Republican Elizabeth Childs, of Brookline, a former state mental health commissioner under former Gov. Mitt Romney, has also announced her intention to run for the GOP nomination.

Kennedy planned to release a video announcement on his campaign website Thursday morning before heading out on a campaign swing through the district.

Kennedy is scheduled to start the day greeting commuters at the Newton Center Green Line MBTA Station before making stops in Milford, Attleboro, Taunton and Westport.

The Kennedy family has seen its influence in Washington fade in recent years as its younger generations have largely shunned public office.

The death of Massachusetts' U.S. Sen. Edward Kennedy, Joseph Kennedy's great-uncle, in 2009 left a void for the family. The retirement of his son, Rhode Island's U.S. Rep. Patrick Kennedy last year marked the first time in 63 years that a Kennedy wasn't serving in elected office in Washington.

Joseph Kennedy III is a graduate of Stanford University, where he studied management science and engineering and was co-captain of the lacrosse team, and of Harvard Law School, where he served as technical editor of the Harvard Human Rights Journal. He served in the Peace Corps in the Dominican Republic from 2004 to 2006.

Kennedy has not held elective office, but he and his twin brother, Matt Kennedy, served as co-chairmen for Edward Kennedy's final re-election campaign in 2006.

Joseph Kennedy III has served as a prosecutor for the Cape and Islands district attorney's office and more recently as an assistant district attorney in the Middlesex district attorney's office, a job from which he resigned last month.

Last month, he also announced he's engaged to be married to Lauren Anne Birchfield, a fellow Harvard Law School graduate.

Other Democratic candidates in the district, which stretches from the Boston suburbs to Fall River, include Paul Heroux, an Attleboro businessman; Herb Robinson, asoftware engineer from Newton; and Jack Porter, a Newton sociologist and real estate consultant.

The 71-year-old Frank announced in November that he would not seek re-election after serving for more than three decades in Congress. In part, the prospect of running in a newly redrawn district with 325,000 new constituents prompted the decision.

Massachusetts has had to redraw its congressional map from 10 districts to nine districts after it lost a seat as a result of the last federal census tally.